If you are 100% positive that this is what's happening, you have legal recourse as blackballing is illegal. However, you had better be very, very sure and be prepared to back it up or it's going to backfire on you. Don't just assume that this must be happening.
A great many people have written to this and other boards absolutely positive that their former employer was blackballing them and that was why they weren't getting a new job, and that wasn't the case at all. They were not getting new jobs because the economy has shifted; because there were more qualified applicants than they; or for any number of other reasons that had nothing to do with blackballing. I've been a hiring manager for years and no one has EVER called me to tell me not to hire this or that person. It happens far less frequently than people assume.